Maybe swab the 6n137’s and try again?
Whoa wait a second - I remember something about some versions of the optocoupler that didn’t work with certain interfaces, due to being too slow… Can’t remember what the fix was though, something about a smaller capacitor or so…
6N138 might be too slow with the 10k pull-up.
No problem with the 6N137, though…
could it be the quality of the midi signal differs from both sources?
if you happen to have another opto, can’t hurt to try.
yeah i seem to remember, could have been a cap or resistor change, i think that was if you wanted to use a 6N138 maybe.
otherwise, maybe there’s a somewhat weak solder joint, good enough for one, but not for the other signal?
far fetched… i know